The invention is primarily directed to improving the durability of exhaust valves and valve guides in internal combustion engines, and is especially well suited for use in large industrial combustion engines fueled by natural gas. Large natural gas internal combustion engines are typically used to produce electrical power or propel ships, etc. Exhaust valves in these large engines are expected to last from 15,000 to 20,000 operating hours. Some manufacturers have gone to great lengths to provide sophisticated lubrication systems to improve longevity of exhaust valves.
Large internal combustion engines typically have exhaust valves covering the exhaust ports which are opened to allow exhaust from the cylinders to escape through the exhaust ports into the engine exhaust manifold. Each exhaust port typically passes through the cylinder head. Each exhaust valve consists of a valve head which covers the exhaust port, and a perpendicular exhaust stem. The exhaust valve stem is supported radially within a valve guide that is either cast into or pressed into an opening in the cylinder head. There is normally only a small clearance between the bore of the valve guide and the valve stem (e.g., 3.2 to 4.3 thousandths of an inch clearance). The small clearance allows very little wobble, but is enough to maintain an oil film to keep the stem from sticking within the bore given manufacturing tolerances and thermal expansion which can occur.
To improve the durability of the interface between the valve stem and the valve guide bore, it is common to use a chrome or other hard, high temperature coating on the stem. Chrome coatings resist wear well. However, it is still desirable to maintain an oil film on the interface of the sliding surface between the valve stem and the valve guide to prevent metal-to-metal contact which tends to wear both metals. Chrome coatings for exhaust valves are not particularly well suited for maintaining a sufficient film of lubricating oil in reciprocating internal combustion engines. This is primarily due to the fact that exhaust valves are stationary almost three quarters of the time that the engine is operating. It is therefore difficult to maintain a hydrodynamic oil film because there is adequate time for the oil film to break down when the valve is not moving. Therefore, lubricating oil is not generally present at all contact areas between the valve stem and the bore in the valve guide when the valve starts to move.
Some coatings are more porous than chrome and help retain oil at the sliding surface. Nitriding is an example of a porous coating. However, even with porous coatings, the oil film can breakdown when the valve stem is not moving, thus allowing metal-to-metal contact at least occasionally. It is therefore desirable to use porous and even non-porous solid lubricant coatings. A solid lubricant coating is a coating having solid lubricity, i.e., the ability to reduce friction is inherent within the solid coating.
Molybdenum and molybdenum oxide are examples of solid lubricant coatings, however, neither can withstand the high temperatures of the harsh environment present in the exhaust port when the exhaust valve opens. The exhaust valve leads a particularly severe life because it is open at a time in the combustion cycle when exhaust gases are approximately 1100.degree. F. or higher. In addition, the hot exhaust gases passing through the exhaust port pass the exhaust valve at a high velocity. In this environment, many coatings are not capable of surviving, including molybdenum oxide coatings as well as other solid lubricant coatings. Although many of these coatings can survive on intake valves, the environment on the exhaust valve is too harsh. These coatings tend to erode rapidly and flake into small particles where exposed to exhaust gas. The small particles can scratch the valve stem, and can possibly get stuck in the exhaust valve seat area which could hold the valve partially open and possibly burn the valve.
Exhaust valve and guide wear has become an important problem, and some manufacturers have even produced sophisticated forced lubrication systems including channels, etc. to extend the life of the exhaust valves and guides. Exhaust valve and guide wear problems are more prominent in natural gas engines because the fuel itself contains no lubricity. Gasoline and diesel fuel have more lubricity in the liquid states, and also form more particulate and ash that can act as a solid lubricant.
It can therefore be appreciated that it would be desirable to provide a cost effective way to reduce the wear of exhaust valves and valve guides, especially in large industrial natural gas internal combustion engines.